Stock-Based Compensation - Stock Options Outstanding and Exercisable (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019 CAD ($) $ / shares shares
$51.17 - $167.50
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	$ 51.17
Range of exercise prices, maximum	167.50
$167.51 - $197.05
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	167.51
Range of exercise prices, maximum	197.05
$197.06 - $247.87
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	197.06
Range of exercise prices, maximum	247.87
$247.88 - $313.16
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Range of exercise prices, minimum	247.88
Range of exercise prices, maximum	$ 313.16
Stock Option Plan
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	1,416,346
Options outstanding, Weighted-average years to expiration	4 years 8 months 12 days
Options outstanding, Weighted-average exercise price	$ 199.12
Options outstanding, Aggregate intrinsic value | $	$ 187
Options exercisable, Number of options (in shares) | shares	654,562
Options exercisable, Weighted-average exercise price	$ 162.59
Options exercisable, Aggregate intrinsic value | $	$ 110
Number of stock options in-the-money | shares	1,416,346
Weighted-average exercise price of stock options in-the-money	$ 199.12
Weighted-average years to expiration	4 years 2 months 12 days
Stock Option Plan | $51.17 - $167.50
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	354,357
Options outstanding, Weighted-average years to expiration	4 years 1 month 6 days
Options outstanding, Weighted-average exercise price	$ 123.00
Options outstanding, Aggregate intrinsic value | $	$ 74
Options exercisable, Number of options (in shares) | shares	303,455
Options exercisable, Weighted-average exercise price	$ 116.84
Options exercisable, Aggregate intrinsic value | $	$ 65
Stock Option Plan | $167.51 - $197.05
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	355,040
Options outstanding, Weighted-average years to expiration	4 years 1 month 6 days
Options outstanding, Weighted-average exercise price	$ 188.53
Options outstanding, Aggregate intrinsic value | $	$ 51
Options exercisable, Number of options (in shares) | shares	135,532
Options exercisable, Weighted-average exercise price	$ 175.30
Options exercisable, Aggregate intrinsic value | $	$ 21
Stock Option Plan | $197.06 - $247.87
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	376,654
Options outstanding, Weighted-average years to expiration	4 years 9 months 18 days
Options outstanding, Weighted-average exercise price	$ 222.75
Options outstanding, Aggregate intrinsic value | $	$ 41
Options exercisable, Number of options (in shares) | shares	215,465
Options exercisable, Weighted-average exercise price	$ 218.98
Options exercisable, Aggregate intrinsic value | $	$ 24
Stock Option Plan | $247.88 - $313.16
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, Number of options | shares	330,295
Options outstanding, Weighted-average years to expiration	5 years 10 months 24 days
Options outstanding, Weighted-average exercise price	$ 265.23
Options outstanding, Aggregate intrinsic value | $	$ 22
Options exercisable, Number of options (in shares) | shares	110
Options exercisable, Weighted-average exercise price	$ 260.52
Options exercisable, Aggregate intrinsic value | $	$ 0